This invention relates generally to the writing of data on magnetic mass storage media and, more particularly, to an apparatus for controlling the writing of GCR data on magnetic tape, said apparatus employing programmable read only memories (PROMS).
Modern data processing system may require information to be written on magnetic tape in one or more of several known formats; e.g. non return to zero-change on 1's (NRZI), phase encoded (PE), or group coded recording (GCR). Copending U.S. patent application Ser. No. 973,259 filed on Dec. 26, 1978 on Write Control Apparatus describes a write control apparatus for constructing the various formats. However, if the GCR format is being written, additional circuitry is required to construct the actual GCR data record consisting of the actual data preceded by a preamble and followed by a postamble. While the GCR data record format will be somewhat described below, a detailed description can be found in the American National Standards Institute standard ANSI X3.54 dated June 10, 1976.
Since the GCR data record includes data groups, a CRC group, a Residual group and a plurality of subgroups consisting of Termination, Sync, Mark subgroup types, written in a prescribed sequence, it has in the past been necessary to provide a large amount of logic circuitry to produce the required record format. This is not only costly, but greatly adds to the overall complexity of the system.